1. Field of the Invention
The present invention relates to a control system for automatically controlling operations of an excavator, a kind of construction machinery, and more particularly to an automatic control system for automatically controlling actuators, such as a boom cylinder, a dipper cylinder, a bucket cylinder, a swing motor and a pair of travelling motors of the excavator. The system automatically controls the actuators to be optimally operated, thereby improving the operational effect and control of the actuators, and help prevent a sudden safety accident during operation of the excavator.
2. Description of The Prior Art
Conventionally, known excavators include a plurality of actuators actuated by hydraulic power. These excavators comprise a driving motor section including the swing motor for swinging an upper frame including a control cab with respect to a lower frame provided with travelling members, such as crawlers. The travelling motors effect travelling of the excavators. An actuating cylinder section comprising the dipper cylinder drives a dipper stick. The boom cylinder drives a boom. A bucket cylinder drives a bucket. The actuators are controlled by control levers/pedals provided in the cab and handled by the operator in order to efficiently carry out several operations of the excavator, such as excavating operations, surface finishing operations, loading operations, and the like.
However, known excavators have disadvantages which cause fatigue of the operator, deteriorate the operational effect of the excavator, causing the actuators to be broken, and possibly causing a safety accident to occur during operations carried out thereby.
First, the known excavator is provided with a pair of travelling motors actuated by hydraulic fluid distributed from the main hydraulic pumps. Control levers/pedals are used by the operator to control the quantity of the hydraulic fluid flow to the respective travelling motors via directional control valves for each of the travelling motors, in accordance with the handling values of the levers/pedals. Also, the excavator includes a solenoid valve, disposed between the inclination angle control members of the travelling motors and sub-hydraulic pump, and adapted for opening and shutting the hydraulic passage from the sub-hydraulic pump to the inclination angle control members. A select switch selectively applies an electric current to the solenoid valve in order to turn the solenoid valve on. Thus, the operator handles the travelling motor control lever/pedal so that a respective quantity of the hydraulic fluid flow, from the main pumps to the travelling motors , is automatically controlled in accordance with the position of the control lever/pedal. The travelling velocity of the travelling motors is changed in a non-step manner. Also, upon turning on the select switch, the solenoid valve is turned on so as to make a fluid passage between the sub-hydraulic pump and the inclination angle control members of the travelling motors, thereby causing the pilot hydraulic fluid to be supplied from the sub-hydraulic pump to the inclination angle control members by way of the fluid passage. As a result, the inclination angle of the travelling motors is controlled so that the velocity of the travelling motors can be controlled.
In other words, if the select switch is turned off, hereinafter referred to "the first step travelling mode," the solenoid valve is applied with no electric current. As a consequence, the fluid passage between the sub-hydraulic pump and the inclination angle control members is shut, and the inclination angles of the travelling motors are maintained at previously set initial angles. In this case, upon handling the control levers/pedals, the travelling velocity V.sub.T in the first step travelling mode is changed in a non-step manner, in accordance with the handling values of the control levers/pedals, from the lowest value V=0 to the highest value V.sub.1 max as represented in FIG. 3.
On the other hand, if the select switch is turned on, hereinafter referred to "the second step travelling mode," the solenoid valve is applied with an electric current so that the fluid passage between the sub-hydraulic pump and the inclination angle control members is opened, and the inclination angles of the travelling motors are changed by specified angles by the pilot hydraulic fluid of the sub-hydraulic pump. In this case, upon handling the control levers/pedals, the travelling velocity V.sub.T in the second step travelling mode is changed in a non-step manner in accordance with the handling values of the control levers/pedals from the lowest value V=0 to the highest value V.sub.2 max, which is relatively greater than V.sub.1 max, as represented in FIG. 3.
However, upon occurrence of an overload on the travelling motors during travelling of the excavator in the second step travelling mode by turning on the select switch, the travelling velocity is suddenly lost. Such an overload occurs when the load is larger than a maximum allowable load P.sub.2 max, which occurs, for example, when the excavator grade is over 40.degree.. Furthermore, the engine and the main hydraulic pumps may suddenly stop, thereby causing occurrence of a safety accident. Also, the operator has to manually select the travelling mode, e.g., the first travelling mode or the second travelling mode, by handling the control levers/pedals in accordance with a grade of the ground surface. However, the travelling motors of the known excavator in an overload condition cannot be efficiently operated, such that a load occurring on the travelling motors which varies according to the grade of the ground surface may exceed the capacity of the excavator motors. Thus, the known excavator has a disadvantage in that it cannot efficiently travel on a relatively high grade of ground surface.
Second, in case of the known excavator having a conventional brake mechanism for applying the brake to the swinging motor adapted for swinging the upper frame with respect to the lower frame, the operator selectively applies or releases the mechanical braking power for the reduction gears of the swing motor by means of a mechanical brake as needed. However, the known excavator has a disadvantage in that the braking operation for the swing motor cannot be efficiently performed, and the swing motor may be driven in order to forcibly swing the upper frame with respect to the lower frame even the braking power of the brake mechanism is applied to the reduction gears. This may cause the reduction gears of the swing motor to be seriously broken. The known excavator has another disadvantage in that the braking power for the brake mechanism may not endure the torque of the swing section of the excavator, such as in the case of operation thereof on a relatively higher grade of ground surface, so that the swing motor section may swing in spite of appliance of the braking power of the brake mechanism to the reduction gears of the swing motor section, thereby causing a serious safety accident.
Third, the hydraulic circuit of the known excavator is provided with a pair of main hydraulic pumps which are connected in parallel to each half of the actuators. Also, a cross hydraulic passage is cross-connected between two actuators selected from each half of the actuators connected to each main pump and is treated as if it is often overloaded so that the fluid flow output from both main hydraulic pumps can be supplied to an overloaded actuator, which is one of the actuators cross connected by the cross hydraulic passage, at the same time. Hence, the actuator can endure a relatively higher load simultaneously with preventing the main pumps from being overloaded. Especially, a directional control valve for the swing motor is cross-connected to a directional control valve for the boom cylinder by means of a cross hydraulic passage which is provided with check valve and a two port directional control valve and a control lever for selectively controlling the pressure of the pilot hydraulic fluid for the two port directional control valve. The fluid is output from the sub-hydraulic pump. Thus, in case of a synchronous operation of actuators in which the swing motor and the boom cylinder are driven at the same time, the boom cylinder conventionally having a relatively higher load than that of the swing motor in this case, a part of the hydraulic fluid from the first main pump for the swing motor can be supplied to directional control valve of the boom cylinder by way of the two port directional control valve of the cross hydraulic passage. This additional fluid is joined with the fluid flow from the second main pump for the boom cylinder.
In the case of an operation for loading excavated objects to transporting equipment, such as a truck, by simultaneous actuation of the control levers for each of the swing motor and the boom cylinder after accomplishing the excavating operation, the control levers for both actuators may be set to their maximum position at the same time, in order to lift up the boom simultaneously with driving the swing motor at the same velocity. In this case, if the transporting equipment, e.g., the truck, is positioned at about a 90.degree. angle with respect to the plane position of the boom, the boom may strike the body of the truck because it cannot ascend to a position above the truck before the swing motor accomplishes its 90.degree. swinging action. Thus, the control levers for each of the swing motor and the boom cylinder are conventionally handled in several stages in order to prevent such an accident, resulting from striking the truck body with the boom. For example, the lever for the boom cylinder is first handled to raise the boom to a safe height, taking into account of the height of the truck, and the lever for the swing motor is then handled in order to swing the upper frame of the excavator with respect to the lower frame. According to another technique, the control lever for the swing motor is first handled in order to swing the upper frame moving the boom close to the truck, and the lever for the boom cylinder is handled in order to perform the desired loading operation of the truck. Thus, the known excavator has a disadvantage in that the loading operation cannot be smoothly and continuously performed, thereby deteriorating the loading operation and burdening an unskilled operator with a relatively difficult loading operation.
Fourth, as described above the quantity of the hydraulic fluid flow which is to be outputted from the main hydraulic pumps to the directional control valves for the actuators is controlled by the control levers/pedals for respective actuators in order to control the actuating value of each actuator. The control levers/pedals are frequently handled in order to start actuation of the subject actuators or to change the actuating direction. At this time, the flow rate of the hydraulic fluid from the main pumps to the directional control valves for the subject actuators is suddenly changed, thereby generating a mechanical shock between the actuators and the hydraulic fluid supplied thereto.
In other words, the actuators temporarily continue the previous state thereof by virtue of mechanical performance. This continuance may result from static friction in the case of starting the actuation, or inertia force acting on the previous actuating direction in case of changing the actuating direction of the actuator, while the hydraulic fluid for the subject actuators is suddenly supplied from the main pump to the actuators in the case of starting the actuation, or suddenly changes the flow direction thereof in the case of changing the actuating direction of the actuators. Thus, a serious mechanical shock occurs between the actuators and the hydraulic fluid, resulting from a striking of the hydraulic fluid and the actuators.
In an effort to solve the above problem, a skilled operator slowly and minutely handles the control levers/pedals so as to make the initial flow rate of the hydraulic fluid be as low as possible. The skilled operator thus makes an effort to reduce the mechanical shock between the actuators and the hydraulic fluid. However, for the unskilled operator, such a minute handling of the control levers/pedals is regarded as a relatively difficult operation, such that the operational effect of the excavator is considerably deteriorated. Even in the case of a skilled operator, such a minute handling is considered to be a burden, and furthermore the mechanical shock cannot be completely removed.
Therefore, the known excavator has the disadvantage that the useful life of the actuators is shortened, because of the mechanical shock generated on the actuators. Also, the operational effect of the excavator is considerably deteriorated because of the minute handling of the control levers/pedals required to reduce the mechanical shock, as described above.
Fifth, as described above, the quantity of the hydraulic fluid flow supplied from the main hydraulic pumps to each of the cylinder actuators, such as the dipper cylinder, the bucket cylinder and the boom cylinder, is controlled by handling the control levers for each cylinder actuator. However, if the control lever for each cylinder is not positioned at the neutral position, but at an actuating position during such handling of the control lever, a mechanical shock will occur, at an end of the cylinder actuator, between the piston rod of the cylinder actuator and the hydraulic fluid due to the quantity of the hydraulic fluid flow. In an effort to remove such a mechanical shock between the piston rod and the hydraulic fluid, a known excavator is provided with a mechanical shock absorber inside each cylinder actuator. However, such an absorber cannot completely remove the mechanical shock. As a result, the known excavator has a disadvantage in that the operational effect and the durability of the cylinder actuators are considerably deteriorated because of the mechanical shock occurring on the cylinder actuators.
Sixth, the excavator is conventionally used for lifting a heavy load, such as a heavy rock, a heavy container, or the like, by means of the operation members comprising the boom, the dipper stick and a lifting hook instead of a bucket. However, in case of a heavy load having a weight over a predetermined allowable maximum lifting capacity of the excavator, the excavator is occasionally tipped, resulting in a serious safety accident. In an effort to prevent such an accident, a specification of the maximum allowable lifting capacity is provided inside a control cab of known excavators in order to allow the operator to determine whether the excavator is capable of lifting the item to be lifted. However, such a determination of the lifting weight at every lifting operation is considered as a burden, resulting in the operator's unconscious omission of the determination in spite of the possibility of an accident if the excavator tips. Also, the maximum lifting capability of the excavator varies in accordance with the operational conditions, and an adjustable extent of the allowable maximum lifting capability of the excavator cannot be easily determined. Thus, the known excavator has a disadvantage in that the operational effect in the case of the lifting operation is considerably deteriorated due to the burdensome determination of the lifting weight at every lifting operation, and furthermore, a serious accident of tipping thereof occasionally occurs due to an erroneous determination of the maximum lifting capability of the excavator.